If you're hoping to qualify for a National Merit Scholarship through your PSAT scores, it would be wise to start studying over the summer. You should take a PSAT practice test first to assess your current score level. You can use the results to decide how much time you want to devote to studying.
If you're below the qualifying threshold for the National Merit Scholarship, work on understanding your mistakes and doing practice questions in your weak areas before taking any more practice tests. If you're already scoring very high, you can just take a few additional practice tests over the summer to get more familiar with the format before you take the real test in October. The PSAT is similar to the SAT content-wise, so you should be able to make a smooth transition into preparing for the slightly longer test in the time between October and January/March.